DE2608669B2 - Device for measuring the optical absorption capacity of a solid, liquid or gaseous sample - Google Patents

Description

2. Apparatus according to claim 1, characterized in that the transmitter is formed by the radiation detector (12).

3. Apparatus according to claim 1, characterized in that the transmitter is formed by a sensor (21) which is independent of the radiation detector (12) is.

4. Device according to one of the preceding claims, characterized in that between Transmitter (12, 21) and comparison device (17) a pulse shaper stage (16) is connected.

5. Device according to one of the preceding claims, characterized in that

a) the alternating current source has an oscillator (13) feeding the actuating device (18), the frequency of which is an integral multiple greater than the desired rotational frequency of the carrier (1),

b) the reference frequency source also has a frequency divider (14) connected to the output of the oscillator (13) for dividing the Frequency of the oscillator (13) by the integral multiple.

6. Device according to one of the preceding claims, characterized in that the carrier disc (1) is arranged in a gas-tight chamber (Xa) that the chamber (\ a) is equipped with windows (Ib) in the wall areas opposite the orbit of the light passage openings and that the pole shoes (4a, Ab or 5a, 5b) of the electromagnetic systems (4,5) are guided through the walls of the chamber (16J) in a gas-tight manner.

The invention relates to a device of the ArL mentioned in the preamble of claim 1

The use of intermittent measuring light beams is in the field of photo-optical Measurement technology generally used. The term "light modulation" is also used, and that for it Devices used are called choppers or »Chopper« also records Die Lichtmodulation occurs primarily for the purpose of separating the actual measurement signal from so-called interference signals, whereby the interfering signal receives a much smaller frequency than the measuring signal while the interfering signal only over fluctuates relatively longer periods of time and can therefore be regarded as a constant signal, the Chopper or chopper frequency selected depending on the measuring device used Semiconductor detectors in the kilohertz range, for Gasde detectors with, for example, membrane capacitors or anemometers in the range of about 5 Hz with an increasing tendency up to about 500 Hz and above.

Light choppers or light modulators for photometers are known in numerous versions

) 5 in most cases around an electric motor on which Shaft a perforated chopper wheel is attached. The measuring effect on the detector of the photometer is in numerous applications of the Chopper frequency dependent, so that usually a syn-

4 () chronmotcr is provided for the drive. There are drives for the chopper disk are also known, in which the motor is a brushless direct current motor is, the speed of which is kept constant by a control arrangement. The speed information that

* "> is used for regulation, is taken from the winding voltage of the motor winding (see DE-OS20 65 118).

In addition, light choppers are previously known in which the modulation of the light by rapidly oscillating

^r > o pendulums or forks, so-called oscillation forks, which are excited in their natural frequency. Deficiency of such devices is due to the size and the fact that when the chopper disk is driven by a motor, the beam path

5 "> almost always partially with the engine compartment in Is connected, which is due in particular when used in gas analysis devices from outgassing effects disturbances of the measurement signal can occur. In some known devices the phase signal is derived from the position of the chopper wheel by means of a so-called resolver such as a light barrier. The phase signal can also from the Drive voltage or, with brushless DC

b "> current motor can be extracted from the motor winding voltage. Phase rotations that occur in the detector and the subsequent amplifier are not taken into account in this measure and can therefore lead to

lead to a negative influence on the measurement accuracy.

It must be taken into account that the measurement signals, which are obtained, for example, from high-resolution photometers at the detector, are very small in the ratio λΐι to the interfering signals that always occur, so that a phase-controlled rectification is usually used for signal separation. Of measuring devices of the type under discussion is generally e ^; ne high resolution expected that a high sensitivity in the response sensitivity ιυ smallest changes in the Meßprobenkonzentrationen down to 10 ~ ² ppm.

An improvement in the evaluation of the measurement signal is possible with frequency-independent detectors by increasing the chopper frequency. Par- π particular, the known sensitivity of gas detectors and signal transmission elements to mechanical shock is thereby greatly reduced At high chopper arise with a motor drive very fast large Schcibendurch- knife for the chopper, since the speed of the motors are gehaiten relatively low due to storage problems and Auswuchtaufwand got to. However, large chopping disks inevitably have undesirable effects in terms of size. r>

From DE-AS 2132 973 it is already known to use the disc-shaped rotor of an electric motor directly as a chopper of a photometric system. Such a solution, however, requires the development of a rotating field and thus the presence of motor windings with which the development of such a rotating field is possible. The speed control of such a system is just as imprecise or complicated as in the known systems in which the motor rotor and chopper disk are designed as separate elements and are only arranged on a common shaft. However, the main disadvantage of such systems lies in the fact that the Maxiamaldrehzahl the chopper w limited by the frequency of the rotating field, and is not sufficient for the desired high accuracy of measurement.

The subject of German patent 25 11 771 is a Gas analysis device for determining the blood alcohol content, in which a chopper, which consists of a chopper disk and a motor connected to this by a shaft, via a speed stabilizer is controlled. So they are largely the same. Disadvantages associated as with that Prior art already outlined: The motor winding tends to outgass; a scheme of Speed is imprecise and sluggish, taking into account that the moment of inertia of the motor is the one the chopper disk can easily exceed 100 times. A regulation of the phase position is with not possible with the known means.

The invention is therefore based on the object of providing the device according to the preamble of claim 1 to be improved in such a way that even with increased chopping frequency and low construction costs a w) extraordinarily high constancy with regard to the speed and at the same time the phase position between the reference signal and measurement signal is achieved.

The solution of the problem is accomplished according to the present invention with those specified in the characterizing part of t>^r> claim 1.

Adjustable eddy current drives with a rotatable metal disc and two encompassing this metal disc AC agents are also known by the term "Ferraris drive". The peculiarity in the subject matter of the invention, however, lies in the structural union of the chopper device with one Ferraris drive and its favorable control behavior. Due to the phase shift between the two alternating currents, the eddy currents can be used to exert a drive torque for the metal disc, which with a phase shift of +90 degrees a torque in a (positive) direction of rotation generated, with a phase shift of 0 degrees a torque of 0 and with a phase shift of - 90 degrees a torque in the opposite (negative) direction. Through continuous change the phase position between the specified values can produce an extremely sensitive and precise torque or torque. Speed control can be effected.

With a constant phase position, a comparable influence is also due to a corresponding change the amplitudes of the excitation currents to each other possible, with a reversal of the direction of rotation or a Braking can be achieved by a negative amplitude of an excitation current. As a metal disc an aluminum disk is used with particular advantage for such eddy current drives has preferred properties.

Further developments of the invention are the subject of the subclaims.

The solution according to the invention allows an extremely exact one with extremely low construction costs Speed control of the metal disc while precisely maintaining the phase position between the reference signal and the measurement signal, so that the chopping device does not falsify the measurement result can occur more or be reduced to a minimum. The volume of the chopper can be kept extremely small even with a gas-tight encapsulation, so that universal application possibilities are created by the subject matter of the invention.

The invention is preferably used in infrared gas analysis.

Embodiments of the subject matter of the invention, their details, mode of operation and others Advantages are shown below with reference to FIGS. 1 to 3 described in more detail.

It shows

F i g. 1 is a perspective view of an eddy current drive composed of a carrier disk and two alternating current magnet systems,

2 shows an axial section through one in one Carrier disc housed in a vacuum chamber and through an associated measuring section and through one of the both AC magnet systems and

Fig. 3 is a perspective view of the object according to FIG. 1 in connection with a block diagram of a control circuit for controlling the speed of the Carrier disc.

In Fig. 1, an existing aluminum carrier 1 is shown, for example, a Has a thickness of about 0.5 mm and is rotatably mounted in bearings 3 via an axis of rotation 2, of which only the upper bearing is visible. It is about a precious stone storage, as it is from the precision engineering is known. In the carrier disk 1, several, for example, are uniformly distributed over the circumference six, light passage openings 9 arranged in connection with the parts located between them

cause the actual light chopping of the carrier disk with a frequency that results from the product of the disk speed and the number of light passage openings. The device shown is designed for a chopper frequency between approximately 50 Hz and approximately ^r < 500 Hz. However, it can easily be used for the smallest frequencies of a few Hz and frequencies in the KHz range.

The carrier plate 1 is assigned two alternating current magnet systems 4 and 5, the essential part of which are ι ο magnetic cores, which are either low-loss ferrite cores or laminated iron cores. The magnetic cores have pole shoes 4a and Ab (FIG. 2) or 5a and 5b, which each enclose an air gap 8 between them. There is a winding 6 or 7 on the yoke of each magnetic core. in particular the radial area between the axis of rotation 2 and the light passage openings 9 should be understood.

The windings 6 and 7, AC currents are fed, having according to the above embodiments, a predetermined or variable relative Phasenla- 2> ge. Due to the spatial arrangement of the carrier 1 and the alternating current magnet systems 4 and 5, eddy currents are induced in the carrier, which, according to the above statements, generate torques by which the carrier can be set in rotation and / or braked.

In the case of the object according to FIG. 2, the carrier disk 1 is surrounded by a disk-shaped, gas-tight chamber la, the cross section of which - as shown in FIG. 2 can be seen - is essentially adapted to the cross section r> of the carrier disk 1 including its bearings 3. The gas-tight chamber la is provided in the area of the orbit of the light passage openings 9 with windows ib , on both sides of which a measuring arrangement is provided, which consists of an optical radiation source 10, an absorption path 11 and a radiation detector 12. The absorption section 11 can also be arranged together with the radiation detector 12 below the carrier disk 1. The absorption section 11 is at Gasanalysenge- ti rates in general a measuring cuvette .sogenannte the prior art. The beam 10a emanating from the radiation source 10 penetrates the sample located in the absorption path 11 and subsequently strikes the radiation detector 12, which is aligned with the characteristics of the radiation. The light passage openings 9 arranged in the carrier disc 1 allow the beam 10a to pass periodically, so that, due to its pulsating intensity, it causes an alternating voltage in the radiation detector 12. The pole shoes 4a and Ab and analogously of course also the pole shoes 5a and 5b are through the walls of the chamber la passed through to the immediate vicinity of the carrier 1 so that the effect described above occurs

In Fig. 3 are the same parts as in the previous ones Figures are provided with the same reference numerals. The carrier 1, which is out of balance with a between driving force and frictional resistance is rotating, is shown with the Control arrangement adjusted to a specified speed for generating a reference frequency An adjustable oscillator 13 is provided for »/ «./«.

whose output frequency generates an integer multiple »n ■ in./« of the reference frequency. This frequency is divided by "n" by a frequency divider 14 and reduced to the reference frequency "/" first./ "The output frequency" n · f _K m of the oscillator 13 results from the optimal drive frequency for the eddy current drive and is the one shown Exemplary embodiment between 700 Hz and 1500 Hz. In a specific case in which a chopper frequency of about 215 Hz was desired, the value "n" was chosen to be 4, so that the drive frequency for the eddy current drive was 860 Hz As a result, the frequency of the chopped beam, known as the chopper frequency, has a fixed ratio to one another in the regulated state, which is given by the value “n.” The oscillator 13 and frequency divider 14 together form a reference frequency source.

The radiation detector 12 is also the transmitter for the control process. For this, the detector signal generated by the radiation detector 12 and amplified by an amplifier 15 is fed via a phase-controlled rectifier 24 to a display or evaluation device A for evaluating the measurement result. In parallel with this, the detector signal, which has the sine-like shape shown, is fed to a pulse shaper stage 16, which is, for example, a Schmitt trigger and converts the sine-like signal into a sequence of square-wave pulses. These are also used to control the phase-controlled rectifier 24, to which they are fed via a line 25.

The output of the pulse shaper 16 as well as the output of the frequency divider 14 to a comparison device 17 is supplied which has the property free by frequency deviations between "f _re i", and the chopper frequency and in phase differences between "" and the measurement signal that may occur despite frequency equality, to deliver an output signal "u" proportional to the deviations, as shown schematically in the signal diagram behind the comparison device 17 The manner described below: If the "u" is very large, which means that the chopper frequency is too low, the eddy current drive generates a high drive force in that the excitation currents in the windings 6 and 7 receive a phase shift of approximately +90 degrees to each other lten. If the value of "u" is too low, which is synonymous with a too high speed of the carrier plate 1, the phase modulator shifts the excitation currents in the windings 6 and 7 in such a way that the phase shift is about -90 degrees a counterforce is generated by which the carrier disk 1 is braked. If the chopper frequency is equal to the reference frequency "/ _re /" and there is no phase shift between the two signals, the phase shift of the excitation currents is so great that precisely the required drive torque is generated. In all control processes, the excitation currents in the windings 6 and 7 are approximately constant and only their phase relationship to one another is influenced as described above

supplies the current amplification required for eddy current propulsion. Oscillator 13 and control device 18 together form an alternating current source for supplying the alternating current magnet systems 4 and 5.

The control method described above is only suitable for those methods in which the Measurement absorption 0 in the absorption section U (Fi g. 2) a usable output signal is present at the radiation detector 12. But this is the case with the one shown Arrangement of the case.

In the case of the in FIG. 3, the measurement signal present at the output of the amplifier IS is transmitted to the display and evaluation device A at the same time

and fed to the control loop itself. However, it is also possible to use a signal transmitter other than the radiation detector 12 for generating a speed signal Use carrier 1, such as a magnetic pickup 21, which is not shown with Magnet bodies cooperate in the carrier disk 1 and also forms a transmitter. In this case it will the detector signal is fed to the control arrangement via the dashed line 22, and the original Line is interrupted at point 23 so that the measuring circuit and control circuit are electrically separated from one another.

For this 2 Biaii drawings

Claims (1)

Patent claims: 1. Device for measuring the optical absorbance of a solid, liquid or gaseous sample with a) an optical radiation source for exposing the sample to a beam of rays, b) a chopper device for the periodic interruption of the measuring beam, which has a rotatably mounted chopper disk provided with light passage openings and includes a speed-controlled drive system, as well as c) a radiation detector for the measuring light influenced by the sample, marked by d) one forming the chopper! Ferrari motor (1 to 8), which comprises a metallic carrier disc (1) functioning as a chopping disc and two electromagnetic systems (4,5), each on opposite sides of the carrier disc (1) outside the area of the light passage openings (9) and the axis of rotation (2 ) have arranged pole shoes (4a, 46 or Sa, 5b) , e) an alternating current source (13, 18) for feeding the two electromagnetic systems (4, S) with alternating currents out of phase with one another, f) a transmitter (12, 21) for generating a signal with the actual frequency of the carrier («Λ g) a reference frequency source (13, 14) for specification a signal with a target frequency, h) a comparison device (17) for comparing the actual frequency with the target frequency, i) an adjusting device contained in the alternating current source for setting the phase position or the amplitude ratio of the alternating currents feeding the electromagnetic systems (4, 5) as a function of the output signal the comparison device (17).